1. Field of the Invention
This invention relates to a method and apparatus for detection of an acoustic alarm signal. More specifically, a band-pass filter emphasizes the dominant frequency of an audible alarm, and a controller uses a tallying algorithm to detect the temporal pattern of the alarm.
2. Description of the Prior Art
Audible alarms are commonly used for many purposes, such as warning of dangerous conditions, indicating when some process has completed, or annunciating the need for some action or intervention. Usually, such alarms are constructed with the intent of being perceived and recognized by humans. For example, a smoke detector is intended to warn people of the potential danger of fire.
In some cases, it is desirable for a machine to react to an alarm signal, without human participation. For example, a sprinkler system may be activated automatically by the signal from a fire detector.
A straightforward approach to such direct activation is to establish a direct electrical connection between the alarm source (“detector”) and the system intended to react to the detector's output. Some detectors are equipped with electrical contacts, which open or close depending on the detector's output state. These contacts may be wired to- and monitored by a separate system provided to react to the output of the detector. However, such direct connection requires special equipment or features within the detector, as well as dedicated installation of wires between the detector and the reacting system.
Therefore, it is desirable to produce a system capable of responding directly to the audible output of certain alarm systems. One application is to provide luminous- or mechanical stimuli to deaf persons in the event of a smoke detector issuing an alarm. This topic is discussed in “Smoke Detector Alarm for the Deaf”, Final Report for Phase II SBIR contract under NIH grant 2R44 DC004254-2, which is included herein by reference as reference 1. Reference 1 discusses a system wherein a microphone's output is processed by a computer program including a Fast Fourier Transform (FFT) to discern the dominant frequency of a smoke alarm conforming to the specifications given in ISO 8201, which is included herein by reference as reference 2. FIG. 1 is a block diagram of the system of reference 1. Microphone 1 converts the acoustic alarm to an electrical signal which is amplified by amplifier 2. The output of amplifier 2 is fed to a high-speed analog-to-digital converter (“ADC”) 3, which samples its input at a rate much higher than the highest frequency of interest, for example approximately two- to five times the nominal fundamental frequency of the alarm to be recognized. The output data from ADC 3 is fed to a computer 4, which uses a FFT algorithm to compute the frequency content of the original acoustic signal. The FFT results are further processed by a temporal-pattern recognition algorithm, to detect the presence of an alarm signature.
Implementation of the aforementioned algorithm requires high-speed analog-to-digital conversion (“ADC”) as well as a fast and powerful computer to perform the FFT calculations. So, construction of a system based on the disclosed technology will be relatively expensive and un-suitable for low-end consumer applications.
Smoke detectors and other alarm-issuing equipments are available on the consumer market at low cost. However, other systems of similarly low cost, with capability of responding to these consumer alarms, are not available. What is lacking in the art, therefore, is an electronic system that is capable of reliably detecting a particular audible alarm at low cost.